One of the issues that can arise if you use analog interfaces on a VoIP system is echo. Echo is simply what you say being reflected back to you a short time later. The echo is caused by the far end, but you are the one that hears it. It is a little known fact that echo would be a massive problem in the PSTN were it not for the fact that the carriers employ complex (and expensive) strategies to eliminate it. We will talk about echo a bit more later on, but with respect to hardware we would suggest that you consider adding echo cancellation hardware to any card you purchase for use as a PSTN interface. While Asterisk can do some work with echo in software, it is usually not nearly enough power to deal with the problem. Also, echo cancellation in software imposes a load on the processor; hardware echo cancellers built into the PSTN card take this burden away from the CPU.

Hardware echo cancellation can add several hundred dollars to your equipment cost, but if you are serious about having a quality system, invest the extra money now instead of suffering later. Echo problems are not pleasant at all, and your users will hate the system if they experience it.

As of this writing, several software echo cancellers have become available. We have not had a chance to evaluate any of them, but we know that they employ the same algorythems the hardware echo cancellers do. If you have a Digium analog card of recent purchase, you can call Digium sales for a keycode to allow their latest software echo canceller to work with your system^[35]. There are other software options available for other types of cards, but you will have to look into whether you have to purchase a license to use them^[36]. Keep in mind that there is a performance cost to using software echo cancellers. They will place a measureable load on the CPU that needs to be taken into account when you design a system using these technologies.